1. Field of the Invention
The invention relates to plants that hyperaccumulate cadmium and zinc and the use thereof to recover cadmium and zinc from soil.
2. Related Art
Industrial practices such as mining, smelting, and disposing of manufacturing wastes have increased the concentration of toxic metals in the environment. For example, at many zinc mining and smelting sites, levels of zinc and cadmium in soil have become so high that few plants survive, resulting in severe disruption of local ecosystems. Once zinc and cadmium enter the soil, their removal is difficult since they are relatively immobile, and do not degrade into less toxic substances. The size of areas affected by smelter and mine wastes are usually so large that engineering methods of soil remediation, such as soil removal and replacement, are too expensive to be practical (Cunningham et al., Trends Biotechnol. 13:393–397 (1995)).
The ability of certain plant species to grow in metal-contaminated soil, and to actively accumulate heavy metals in their tissues, has created an interest in using such plants to extract metals from soil to recover the metals and/or to decontaminate the soil. For example, hyperaccumulators can be used to reduce the level of cadmium in rice paddies deposited from mine wastes. Prolonged consumption of rice grain produced on contaminated fields can harm human and animal health even if the cadmium concentration is low. A concentration as low as about 1.0 ppm Cd can cause harm if the rice is grown on soil with little ability to adsorb Cd. In addition, high levels of soil metals deposited by, for example, an industrial accident, can be removed using hyperaccumulators. Such removal would be economically feasible.
Growing plants, including crops, on contaminated soil to extract contaminants is referred to as phytoextraction. This method is particularly effective in arable contaminated soils because it causes little disruption or dispersal, while preserving soil fertility and landscapes.
It has long been known that certain types of soil and geological materials, including serpentine, lateritic serpentine, ultramafic and meteor-impacted soils are rich in nickel and cobalt and other metals. These soils can be conventionally mined or cultivated with metal-accumulating plants. Using plants to extract metals from such mineralized (geogenic) soils is referred to as phytomining.
Thlaspi caerulescens (alpine pennycress), a non-crop member of the Brassicaccae family, is zinc- and cadmium-tolerant and can accumulate exceptionally high levels of both metals in its shoot tissue. However, the usefulness of T. caerulescens for soil remediation is thought to be limited by its small size (about 15 cm high), slow growth rate and rosette growth habit which would make mechanical harvesting difficult. Dry weight yield over a 6-month growing season has been estimated at 5 t ha−1 (Chaney et al., Current Opinion in Biotech. 8: 279–284 (1997)). Based on the results of preliminary greenhouse and field studies, the time required for phytoremediation of zinc-contaminated soils using T. caerulescens has been estimated to be between 13 and 28 years.
Brown et al., Environ. Sci. Technol 29:1581–1585 (1995), performed a 2-year field study in which T. caerulescens, Silene vulgaris (bladder campion, a zinc-tolerant non-hyperaccumulator) and lettuce were grown to maturity or for 2.5 to 4.5 months on plots which had received three different biosolids treatments at least 13 years previously. The pH of each plot was adjusted to two levels (about pH 5.0 and about pH 6.5) such that full plots existed in the field for lower and higher soil pH. Three replications of each plot were cropped for the study. The metal contents of the bio solids-treated soils were 119, 144 and 181 mg/kg Zn and 1.0, 3.0 and 5.5 mg/kg Cd, respectively. Shoot zinc concentration was highest in T. caerulescens with a maximum of 4440 mg/kg. The cadmium concentration of T. caerulescens, which reached a maximum of 28 mg/kg on the soil with the highest metal concentration and the lowest pH, was not significantly different from that of lettuce, but was higher than that of S. vulgaris (18 mg/kg Cd). However, the authors suggested that S. vulgaris may be the better choice for phytoremediation of cadmium because, although it accumulated a lower concentration of cadmium in its shoot tissue than T. caerulescens, the more vigorous growth of S. vulgaris would make it easier to establish and harvest.
Baker et al., “In situ Decontamination of Heavy Metal Polluted Soils Using Crops of Metal-accumulating Plants—A Feasibility Study,” in In Situ Bioreclamation: Applications and Investigations for Hydrocarbon and Contaminated Site Remediation, R. F. Hinchee and R. F. Olfenbuttel (eds.), Butterworth-Heinemann, Boston, Mass., pp. 600–605 (1991), conducted a greenhouse study in which three hyperaccumulators (Thlaspi caerulescens, Alyssum lesbiacum and Alyssum murale) and three non-hyperaccumulators (Brassica oleracea (cabbage), raphanus sativus (radish) and Arabidopsis thaliana) were grown for 5 weeks on soil to which high-metal sewage sludge had been applied from 1942 to 1961, resulting in a metal content of 380 mg/kg Zn and 11 mg/kg Cd, both in excess of European (EEC) regulatory levels, i.e., 300 mg/kg Zn and 3.0 mg/kg Cd. However, the hyperaccumulator plants did not accumulate economically useful levels of metals from the contaminated soil. For example, the zinc content in T. caerulescens leaves was about 2000 mg/kg dry weight and the cadmium content was about 20 mg/kg dry weight.
In a field study, Baker et al., Resources, Conservation and Recycling 11:41–49 (1994), grew six hyperaccumulator species, including two populations of T. caerulescens, and three non-accumulator species, including Brassica napus (rapeseed) and radish, for 5–6 months on sludge-polluted soil containing 444 mg/kg Zn and 13.6 mg/kg Cd, pH 6.6. The maximum zinc concentration in the above-ground biomass of T. caerulescens was 6500 mg/kg. T. caerulescens had the highest rate of zinc removal from the soil extracting 30 kg/ha zinc.
In view of the results obtained with known zinc and cadmium hyperaccumulating plants, it would be desirable to have larger and/or more vigorous hyperaccumulator plants that could remove more metals from soil more efficiently for phytoextraction for value and for soil decontamination wherein metal recovery from the plants is not cost effective, but the process effectively removes soil contaminants. Generally, for phytoremediation and/or phytoextraction, the soil contains greater than about 1.0 ppm to about 10,000 ppm Cd and/or greater than about 300 ppm to about 100,000–150,000 ppm Zn.